1. Field of the Invention
The present invention relates to display apparatus, and more particularly relates to apparatus for reducing unwanted magnetic radiation external to a cathode ray tube display device without affecting the intended deflection field within the bore of the yoke.
2. Background Art
Cathode Ray Tubes ("CRTs") generally have associated coils, or yokes, to provide a varying magnetic field for electron beam deflection, for example for raster scan. In addition to manifesting itself within the CRT, for beam deflection, this magnetic field also extends around the outside of the CRT and beyond the display device. This external magnetic field serves not useful purpose and an effort is frequently made to reduce this part of the yoke magnetic field. In particular the unwanted frequency range is from 1K to 350K hertz (VLF).
A. A. Seyno Sluyterman of Phillips describes the radiated field due to the horizontal deflection system in his paper entitled "The Radiating Fields of Magnetic Deflection Systems and Their Compensation" presented in 1987 SID Society of Information Display Proceedings. In that paper it shows that the radiated field of the horizontal magnetic circuit of the yoke at mid-range, resembles a vertically oriented dipole, whose mathematical center lies on the long axis slightly ahead of the yoke,
Means to provide reduction of this radiation are proposed in this paper. In one case the Helmholtz coils are "on top" and "below" the saddle-shaped deflection yoke. In another case the Helmholtz coils are behind the yoke. The coils are coupled to the deflection coils and the EMF is induced therein, giving rise to a magnetic field which tends to cancel the unwanted radiated magnetic field. However, this is a relatively expensive and bulky solution to the problem. A similar top and bottom coil configuration is in published Finnish Patent Application No. 861458, Apr. 4, 1986 of Nokia.
Another proposed solution is the placement of shielding all around the CRT, which results in magnetic radiation reduction from the eddy currents induced in the shielding. However, this is also an expensive solution to the problem, and results in only minimal reduction in the magnetic field in front of the screen.
Accordingly, there is a need for means to reduce to acceptable levels the residual magnetic field in front of the cathode ray tube display device that provides an inexpensive and compact solution to the problem.